Load responsive systems have been used in the past to improve the efficiency of fluid systems and also to improve the quality of the controls in the systems. One of the basic controls of the load responsive system is the load responsive pump control, which is made responsive to the largest of the system loads and automatically maintains a relative constant pressure differential across a variable orifice of a control valve interposed between the pump and the largest load. Systems today normally require fast response of the control and therefore fast response of the load responsive pump control. In many systems having large flow requirements at high pressure, a variable displacement piston pump is used. These variable displacement pumps, especially those delivering high flows, are characterized by the large inertia of their displacement changing mechanisms. In the case of a cantilever piston type pump having a swashplate, the inertia of the displacement changing mechanism consists not only of the large inertia of the swashplate but also half of the total inertia of all the working piston assemblies in contact with the swashplate. The cantilever piston type pump is the most commonly used in both variable displacement system pumps working in the range of high pressures and the majority of hydrostatic transmissions. In order to provide a reasonable response of the flow changing mechanism of such a pump, as a rule, two stage controls are used. It is difficult to obtain a fast response of such a pump control, since the second stage control is subjected to the comparatively low forces resulting from relatively small pressure differentials. When controlling such pumps, it is difficult to obtain fast response due to the large inertia of the pump displacement changing mechanism and it is also difficult to obtain fast response in pumps generating very high flows. Even if the swashplate could be moved quickly from a maximum flow condition to a minimum flow condition, the harmful side effect of cavitation could occur in the pumping chambers and/or the system could be subjected to large pressure spikes during such large flow transients.
The present invention is directed to overcoming one or more of the problems as set forth above.